1. Field of the Invention
This invention relates to a method of treating an electrostatic image in order to electrically charge and develop the electrostatic image that is formed in electrophotographic process, electrostatic recording process, electrostatic printing process, and so forth.
2. Description of the Prior Art
As the systems for developing an electrostatic image to form a visible image, a wet developing system using a liquid developer and a dry developing system using a powder developer are generally known. The latter, which is a dry process and is advantageous because plain paper is usable, is further classified broadly into a system using a two-component developer consisting of a carrier and a toner and a system using a one-component developer consisting of the toner alone.
In comparison with the system using the two-component developer, the developing system using the one-component developer is more advantageous because the change in the toner concentration does not occur in itself, resulting in simplification of a developing means and because the characteristics of the developer does not deteriorate but remains stable over an extended period. However, this system is not devoid of the critical drawback in that a stable image-forming property cannot be obtained. This is because it is difficult to electrically charge the one-component developer in a desired state.
In order to obtain a satisfactory visible image by the dry developing system, it is essentially necessary to electrically charge the toner in a necessary polarity and moreover, in a suitable charge quantity. In the system using the two-component developer, on the other hand, the toner is frictionally charged while the toner and the carrier are being mechanically stirred so that the charge polarity and charge quantity of the toner can be somehow controlled by selecting such conditions as the carrier characteristics, the stirring condition, and so forth. In the one-component developer consisting of the toner alone, however, no such carrier exists and hence control of the charge polarity and charge quantity of the toner is extremely difficult.
As the methods of charging the one-component developer, there have been conventionally known a frictional charging method which agitates the developer by means of a mechanical force, a charge-injection charging method using injection electrodes and a charging method using a corona discharger.
In the frictional charging method, the toner is charged by friction between the toner and the stirrer, between the toner and the vessel wall or between the toner particles by themselves. Accordingly, this method involves the problems that the charge quantity is generally small, control of the charge quantity is difficult and toner is partially charged opposite to the required polarity.
In the charge injection charging method, charge injection becomes difficult if the developer is dielectric, and leak would occur if the developer is conductive, on the contrary, whereby a large charge quantity cannot be obtained.
In the charging method using a corona discharger, there are problems that the developer cannot be uniformly charged and the corona wire is likely to be contaminated.
Thus, it has not been possible in accordance with the conventional methods to charge a one-component developer in a suitable charged state and consequently, extremely strict conditions must be employed in the developing method using a one-component developer. Nonetheless, it has been difficult to stably form a satisfactory visible image and the corona wire has been likely to be contaminated.
On the other hand, various means have been known in the past in order to let the electrically charged one-component developer act upon a support supporting thereon an electrostatic image to form a visible image, by utilizing essentially the electrostatic attraction of the electrostatic image. (The means will hereinafter be referred to as the "developing means".) The developing means are broadly classified into a contact developing system which brings a developer into contact with the entire surface of an electrostatic image support as typified by an impression system, and a non-contact developing system such as a jumping system or a touch-down system in which development is effected while the developer is not brought into contact with the entire surface of the electrostatic image support.
In the non-contact developing system, the toner is caused to jump from a developer support to the electrostatic image support placed so as to oppose the former, thereby effecting development. Accordingly, the toner must have a considerably large charge and in addition, it is essentially necessary that the thickness and surface condition of the toner layer supported on the developer support be uniform.
The contact developing system is preferable because it makes it possible to deposit the toner on the electrostatic image in a reliable manner. Since the developer is brought into contact also with non-image portions where no electrostatic charge exists, however, the toner is highly likely to attach to such portions. It is therefore necessary that the charge quantity of the toner supported on the developer support be uniform. In the impression system or the like, further, the thickness and surface condition of the layer formed by the toner must be uniform. Otherwise the toner attaches also to the non-image portions on the electrostatic image support so that a clear visible image cannot be obtained after all.
In development by use of the one-component developer, may it be the contact developing system or the non-contact developing system, it is desired that the developer is introduced into the developing region, or the region in which the charged one-component developer is permitted to act upon the electrostatic image support, in the minimal quantity required for development. For, unlike the two-component developer, all the one-component developer that have been introduced have the possibility of participating in the development. The minimal necessary quantity means the quantity in the state in which several layers of the developer particles are deposited.
According to the conventional methods, however, it has not been possible not only to obtain the necessary charge polarity and charge quantity for the one-component developer, as described already, but also to introduce the charged developer into the developing region under the desirable state. For these reasons, it has been difficult to stably form the satisfactory visible image.
In the developing system using the two-component developer, the toner and the carrier are mechanically stirred so as to frictionally charge the toner for visualizing the image. Hence, control of the charge polarity and charge quantity of the toner is possible to a considerable extent by selecting the carrier characteristics, the stirring conditions and the like, thus providing a satisfactory visible image. Due to this advantage, the developing system using the two-component developer has gained a wide application in practice.
In the system using the two-component developer, it has been a practice to mechanically stirr the developer in order to electrically charge the developer. For this reason, a stirring mechanism having a large torque is necessary. Moreover, the carrier is likely to be broken and degradation of the developer as exemplified by "toner filming" occurs. Especially when development is carried out at a high speed or when the developing step is continuously repeated a large number of times, these result in the critical problem.